It's easy to design an emulator at the level of the D7220 host interface.
It is _difficult_ to design an emulator at the interface between the D7220
controller and the 7242 Formatter/Sense Amplifier, because the 7242 is a
tricky little beastie, and while the interface is somewhat documented, the
docs aren't terribly clear and not entirely complete, because Intel didn't
think anyone would want to use the 7242 without the D7220.

Unfortunately what Mark needs is to emulate it at the 7242 level, because
the 7242 is in the cartridge and the D7220 is in the host.